Large volumes of commercial acetic acid are produced by carbonylation of an alkyl alcohol, especially methanol, and reactive derivatives thereof, with carbon monoxide in a liquid reaction mixture. Such carbonylation reactions are generally carried out in the presence of a catalyst, often a Group VIII metal catalyst such as rhodium and iridium, a halogen containing catalyst promoter, such as methyl iodide, and water. U.S. Pat. No. 3,769,329 to Paulik et al. discloses the use of a rhodium-based carbonylation catalyst dissolved, or otherwise dispersed, in a liquid reaction mixture or supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. U.S. Pat. No. 3,769,329 to Paulik et al. discloses that water may be added to the reaction mixture to exert a beneficial effect upon the reaction rate, and water concentrations greater than 14 weight % of the reaction mixture are typically used. This is sometimes referred to as the “high water” carbonylation process.
An alternative to the “high water” carbonylation process is the “low water” carbonylation process, as described in U.S. Pat. Nos. 5,001,259 to Smith et al.; 5,026,908 to Smith et al.; and 5,144,068, also to Smith et al. Water concentrations in the reaction mixture of 14 weight % and lower may be used in the “low water” carbonylation process. Employing a low water concentration simplifies downstream processing of the desired carboxylic acid to its glacial form. The more water there is in a reaction stream, the greater the operating costs to remove water from the product acetic acid and the greater the capital investment in product recovery and purification equipment. The efficiencies achieved when operating at very low water concentrations makes it attractive to operate at the lowest water concentration possible.
However, while reducing the reactor mixture water concentration may minimize operating and fixed costs, it is more difficult to maintain catalyst stability and activity, as explained in U.S. Pat. Nos. 5,001,259 to Smith et al.; 5,026,908 to Smith et al.; and 5,144,068, also to Smith et al. In low water acetic acid production, especially in processes using rhodium-based catalysts, the catalyst metals tend to precipitate out of the reaction mixture. Catalyst precipitation is frequently experienced in product recovery systems, especially flasher units. Significant catalyst precipitation may lead to catalyst loss, reduced reaction rates, interrupted unit operation, and complete shutdowns. It is known that catalyst stability problems may be minimized by the use of a catalyst stabilizer such as a soluble metal iodide or quaternary iodide salt. As discussed in U.S. Pat. Nos. 5,001,259 to Smith et al.; 5,026,908 to Smith al.; and 5,144,068, also to Smith et al, especially suitable salts are alkali metal iodides such as lithium iodide since these are the most soluble and thermally stable in the reaction mixture. EP-A-0 161 874 to Smith et al. describes a reaction system in which methanol, is carbonylated to a carboxylic acid derivative such as acetic acid while using a liquid reaction mixture having low water content. The disclosure describes that this is achieved by the use of defined concentrations of an iodide salt, alkyl iodide and corresponding alkyl ester in the liquid reaction mixture to maintain rhodium catalyst stability and system productivity. EP 0 506 240 B1 to Watson discloses the introduction of one or more iodides of Group IA and IIA elements or hydrogen iodide into the flasher zone of an acetic acid recovery system. The introduction of the iodides is said to suppress the volatility of water relative to the acetic acid to aid in the recovery of the acetic acid.
Several patent references disclose the use of ruthenium, osmium, cadmium, mercury, zinc, gallium, indium, and tungsten for use as promoters in iridium catalyst systems. See, U.S. Pat. No. 5,510,524 to Garland et al.; EP 728 726 A1 to Garland et al.; EP 752 406 A1 to Baker et al.; EP 849 249 A1 to Ditzel et al.; and EP 849 250 A1 to Williams. Similarly, U.S. Pat. Nos. 6,458,996 to Muskett; 6,472,558 to Key et al.; and 6,686,500 to Watt and EP 643 034 A1 to Garland et al. mention the use of ruthenium and osmium as promoters for iridium catalyst systems. U.S. Published Patent Application 2004/0122257 to Cheung et al. discloses the use of salts of ruthenium, tungsten, osmium, nickel, cobalt, platinum, palladium, manganese, titanium, vanadium, copper, aluminum, tin, and antimony as catalyst co-promoters with rhodium catalyst systems in acetic acid production systems having less than 2 weight % water. U.S. Pat. No. 5,760,279 to Poole discloses the incorporation of a manganese stabilizer in conjunction with a rhodium catalyst. U.S. Pat. Nos. 4,433,166 to Singleton et al. and 4,433,165 to Singleton and EP 0 055618 to Singleton et al. disclose the use of tin as a rhodium catalyst system stabilizer used in high water carbonylation processes. The English language abstract of the publication entitled Stabilization of Stannous Chloride for Rhodium Complexes Catalyst, Journal of Xiamen University (Natural Science) Vol. 25 No 4 at pg. 488 (July 1986) also discloses the use of tin as a rhodium catalyst system stabilizer. The use of tin as a rhodium catalyst system stabilizer over certain temperature and pressure ranges is disclosed in the publication Zong, Xuezhang, et. al, The Thermal Stability of Rh(I) Complex Catalyst In The Carbonylation of Methanol To Acetic Acid, Southwest Res. Inst. Chem. Ind., Naxi, Peop. Rep. China. Cuihua Xuebao (1982), 3 (2), 110-16. CODEN: THHPD3 ISSN: 0253-9837. None of the references that disclose the use of ruthenium or tin as a rhodium catalyst system stabilizer or promoter disclose also the incorporation of the stabilizer in a low water system including an iodide ion, provided by an iodide salt, at concentrations of greater than 3 weight % of the reaction mixture.
EP 0 728 727 B1 to Poole et al. and equivalent U.S. Pat. No. 5,939,585 to Ditzel et al. disclose the use of ruthenium or osmium as a catalyst promoter to enhance production rates in combination with alkyl halide such as methyl iodide for the production or carboxylic anhydrides and acetic acid. The patent discloses that when carboxylic anhydrides are being produced, the iodide co-promoter may be selected as N,N′ dimethyl imidazolium iodide or lithium iodide preferably present at concentrations up to its limit of solubility, for example 30 weight % lithium iodide. However, when acetic acid is produced, the references disclose that the iodide co-promoter may be lithium iodide but it should only be present at concentrations of less than 3 weight % lithium iodide. Such co-promoters will reduce the formation of volatile promoter species and thereby facilitate product recovery and purification. There is no mention of the use of lithium iodide as a stabilizer but only as a suppressant of volatility. However, the references note that the ruthenium or osmium promoters act as stabilizers for the rhodium catalyst at low partial pressures of carbon monoxide. Experiment “X” of EP 0 728 727 B1 to Poole et al. discloses 90.7% of rhodium precipitated in 23 hours without inclusion of ruthenium or osmium in an autoclave system. Example 33 of EP 0 728 727 B1 to Poole et al. discloses that inclusion of 20 molar equivalents of ruthenium trichloride hydrate per rhodium carbonyl chloride dimmer in the autoclave system reduced rhodium precipitation to 55.6% of rhodium from the solution.
Experiment H of EP 0 728 727 B1 to Poole et al. notes that the addition of lithium iodide to a reaction mixture for the production of acetic acid does not allow the reaction to remain constant. Therefore, as noted in Experiment H, ruthenium or osmium was not added to a reaction mixture containing lithium iodide. Presumably, because of the perceived rate destabilizing effects of lithium iodide in combination with low water conditions, EP 0 728 727 B1 to Poole et al. advises that when the ruthenium or osmium is added in combination with lithium iodide under low water conditions, it should only be done at lithium iodide concentrations of less than 3 weight %.
The publication New Acetyls Technologies from BP Chemicals, Science and Technology in Catalysis 1999, M. J. Howard, et al., pp. 61-68 reports “a non-commercial example” of, as described in EP 0 728 727 B1 to Poole et al. which is referenced in the publication, the use of ruthenium as a promoter to increase reaction rates in low water carbonylation systems using a rhodium catalyst. The use of another promoter, such as an iodide salt promoter, as a catalyst stabilizer, is not disclosed.
Published PCT Applications WO 2004/101487 to Gaemers et al. and WO 2004/101488 to Gaemers et al. disclose processes for production of acetic acid using rhodium and iridium metals coordinated with a polydentate ligand as catalyst systems. The published applications disclose the systems incorporating ruthenium, osmium, rhenium, cadmium, mercury, zinc, gallium, indium, and tungsten compounds as promoters. Molar ratios of the promoter to the rhodium or iridium of 0.1:1 to 20:1 are disclosed. Alkyl halide co-promoters are also disclosed. Additionally, water concentrations of 0.1 weight % to 10 weight % are disclosed. Finally, the published applications indicate that “an effective amount” of a stabilizer and/or promoter compound selected from alkali metal iodides, alkaline earth metal iodides, metal complexes capable of generating iodide ions, and salts capable of generating iodide ions may be incorporated. No specific information regarding the concentration of the “effective amount” is provided. The term “effective amount” is considered to refer to the iodide concentrations disclosed in the representative art as suitable for use in conjunction with ruthenium and tin compound promoters. In other words, iodide salt concentrations of less than 3 weight % are considered to represent an effective amount of the iodide compounds.
In summary, certain references disclose the use of various ruthenium and tin compounds as catalyst promoters and/or stabilizers. However, these references also disclose that the ruthenium and tin promoters and/or stabilizers are to be used only in systems incorporating low levels of iodide salt catalyst co-promoters or in the complete absence of iodide salt co-promoters.